Which? Tests Effects of Low Tyres Pressure

The dangers of driving with low tyres pressure have been highlighted by consumer watchdoge magazine Which?

Researchers took a typical family hatchback – a Peugeot 308 – and deliberately deflated one or more tyres at a time to see how it affected the cars performance.

In every case fuel economy was worse and its anti-lock braking system (ABS) was less effective.

The researchers reduced the pressures of the different tyres from their original setting of 2.4bar (35psi), first by 0.5 bar, then by 1.0 bar and then by 1.5 bar (down to 13psi). As a guide, one bar = 14.5psi (pounds per square inch).

They found that when the right front tyre was deflated, the biggest risk was to how the car gripped the road when a sideways force was applied

After accelerating round a fixed-bend from 37mph to 60mph they measured how much sideways slip there was.When the pressure in the right front tyre dropped by just 0.5 bar, the cars ability to keep its line was reduced by 7%. When the pressure dropped by 1.0 bar, it lost one third of its lateral grip, although further reduction made it no worse. The car was also slower at making an emergency lane-change (in the dry).

A drop of 1.5 bar brought the maximum swerve speed down from 44mph to just over 37mph.

Fuel use increased a little – by about one or two miles a gallon.

When the right rear tyre was deflated there was has far less effect on safety, with one exception.

In a situation such as when you have to change lanes in an emergency where the load is suddenly transferred on to the back wheel, a reduction of 1.5 bar led to diminished lateral grip, reducing the maximum test speed from 44mph to about 39mph.

When all the tyres were deflated the performance was generally similar to having low pressure in the right rear tyre, although the ABS was more effective. There was a similar drop in how quickly you could change lanes while still keeping control of the car, as was seen with one tyre deflated.